1. Field of the Invention
The invention relates to a continuous process by which waste waters containing low to high concentrations of organic constituents and total nitrogen (definition as given in H 12, the German standard method for the examination of water, waste water and sludge) are biologically purified to such an extent that the said organic constituents are minimized, the said content of total nitrogen is completely nitrified, if it exceeds the assimilation demand, and the nitrate formed is reduced.
In this process, the expenditure for the oxygen introduction, the expenditure for the denitrification and the quantity of excess sludge are minimized.
2. Description of the Prior Art
It is known that, when waste water is treated in an upstream denitrification stage, substantial proportions of the oxygen which serves to oxidize the nitrogen compounds can be recovered by recycling activated sludge from the settling basin of the activated sludge plant and/or by recycling waste water from the nitrification basin [1] [2] [3] [4]. (The numerals refer to the references listed at the end of the specification.)
It is furthermore known that the load of organic constituents flowing into the first stage is reduced by said denitrification and that a low BOD.sub.5 sludge loading is necessary for a nitrification [1].
It is furthermore known that a complete denitrification of the water fed to the first stage is only possible if the COD feed/NO.sub.3 -nitrogen recycle ratio corresponding to the stoichiometry is 4 kg/kg or higher [2]. In the case of industrial waste waters, in particular, the ratio of organic load and total nitrogen is, however, often subject to large variations. If, during these variations, the COD feed/NO.sub.3 -nitrogen recycle ratio exceeds the above-mentioned amount of 4 kg/kg, the excess COD has to be biodegraded in the nitrification stage.
Surprisingly, it has now been found that it is possible to achieve both a complete denitrification and also a substantial biological reduction in the excess organic constituents by a controlled aeration of the first (denitrification) stage.